RC Powered PWC?

[rbwood]

Hello,

I've been working on a mini boat project, and am having a little trouble figuring out how to power the thing. Most people just power them with Trolling motors, but trolling motors all cap out at 5mph and I want to hit closer to 10mph.

I've been wondering about doing an RC Setup, maybe with dual jet pumps for 360 steering and extra thrust. I've been reading a lot of forums and how-to sites, but all of them are geared towards little plastic boats flying across the water at 45mph. I'm wanting to get a 7' long, 300lb, flat bottom(for now, looking into doing a tunnel hull later) boat to trudge along at 10 mph min, 20 mph max. I'm thinking a larger prop(but have no idea how large, 25mm? 50mm? 75mm? 100mm? 200mm?), on a lower KV motor, running 3S, or possibly even a deep cycle marine battery (Though I keep reading conflicting things on how you can or can't run RC off lead acid).

Anyone want to help me get this one in the water? Or should I just throw a prop on a powerdrill like the other yahoos?

[NativePaul]

300lb with you, or 300lb plus you? if it is plus you, what do you weigh? 7' is pretty small for a 300lb boat indicating quite a deep draught, and it needs to be at least as wide as your hips, so the length to width ratio is going to be low which has a huge impact on the speed of a displacement boat. 20mph just isn't possible and even 10mph will take so much power it isn't really feasible. How married to the 7' length are you? A canoe/kayak/skiff type hull with the same weight but more than double that length would have a shallower draught so be more practical in many rivers, and require a fraction of the power to push through the water at speed.

Using the boat speed calculator at http://www.psychosnail.com/boatspeedcalculator.aspx assuming it being 300lb including you, a 7' boat will have a hull speed of 3.5 knots, while pushing it faster than that is possible, it means that you are then going uphill on your own bow wave, and it take exponentially more power to go faster. For example 4mph requires 440watts, 5mph requires 1470W, 6mph requires 11.7kw

Doing nothing but doubling the waterline length of the boat to 14' reduces the power required to go 6mph from 11.7kw to 560w

[NativePaul]

In case you are made of money and really want it, the only suitable electric RC jet drive I know of is https://www.mhz-watercraft.com/shop/...n-jet-64?c=110 $3000 each for approx 2mph more, at the cost of a lot of noise, heat, vibration, and 1/27th of the runtime.

[rbwood]

@NativePaul -
Heh, no, definitely not made of money.

It's 300lbs with me in it, the draft on this should be 4-8" based on similar builds I'm using for inspiration(I haven't finalized the design yet, so don't know the exact draft). The bottom of the hull is fairly wide, but the sides also extend out, they aren't just vertical. I can make the hull as narrow as 18" at the waterline, but my current design is 20" towards the bow and 26" at the stern.

I am not married to the flat bottom overall, just for the first boat I build; after that I have an idea for more of a tunnel hull, or catamaran style with twin V hulls.

I have a 55lb trolling motor, which I would think would be more than enough power to get it to the motors ax speed of 5MPH, I thought about maybe using that with a gearbox and a different prop to try to get a little more speed out of it, but I've seen a lot of cautionary tales of possibly burning out the motor (especially if the motor isn't submerged for cooling)

I appreciate the info, and the link to the calculator. Is hullspeed really singularly dependent on length? I would think that a smooth, more rounded hull(still flat bottom, but rounding up to the bow and sides), even half as wide as long, would move much faster and more easily than a more boxy hull like a johnboat, of the same length(perhaps less stable though if you were to stand up).

[Mxkid261]

Just curious, why do you want to go so slow?

[pescador]

There is the Bixpy water jet, it’s an add-on for kayaks, SUP’s and inflatables. But it’s $1000.
https://bixpy.com


Sent from my iPad using Tapatalk

[NativePaul]

Performance is is not soley dependant on length, there are other factors with width and shape and weight being big players too, but length plays a huge part. That calculator only plugs in the length and the weight assuming the width and shape to be typical for the length. I would guess that its output will be optimistic for the boat I am imagining for your description.

Look at a single skull rowing boat, it only has to displace enough water for one athlete in spandex (about 220lb) and itself (a little over 30lb) so is about 50lb lighter, yet to maximise performance they are getting almost 30' long. The combination of long length, narrow beam, low weight and sleek design mean they can do 10mph on the 500w an athlete can produce.

Make it as narrow as you can if your primary objective is performance, and if you can make the front and pack pointed, rounded, or even chamfered, it will be better for performance than just being flat

You might be looking to a planing design for more speed, as hull speeds and sweeping lines are only aplicable to displacement boats and are irrelevant once planing, but even planing boats are displacement boats untill they reach planing speed, and with such a short boat it will take a LOT of power to climb up out of the hole and get onto the plane.

[rbwood]

@pescador
Yeah, I've seen those. They max out at 6mph, and are a bit more than I'm hoping to spend. I'd like to get something set up for under $500.

@NativePaul
I could probably make it narrower at the waterline, at the expense of added draft.
The twin hull deign I'm working on, will be very narrow (probably around 12" at the waterline) and tapered front and back, so maybe that design will be able to get a higher speed with the same power setup as the flat bottom boat.

I don't know that planing will be an option, I'm not sure I'm planning on going fast enough to get up on plane (I imagine most planing hulls don't get up on plane until at least 15-20mph?).

-------------------------------------

Does anyone have any advice to my root question of what kind of setup I should be looking to do (motor, ESC, Prop, # of cells)? If I do a 50mm prop on a 850kv motor running 3S what kind of performance would that give me? If weight and hull length weren't a factor, what kind of power and max speed would this setup have?

I've tried a dozen different calculators, but all of them are for only a couple specific brands of components and are not very helpful.

Thanks!